(a) Technical Field
The present invention relates to a manifold block for a fuel cell stack. More particularly, it relates to an apparatus and method for forming an oxidation layer on a manifold block for a fuel cell stack, which can form an oxidation layer uniformly over the entire surface of a long and complicated internal flow field of a manifold block.
(b) Background Art
A fuel cell is an electrical generation system that does not convert chemical energy of fuel into heat by combustion, but rather electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. At present, one of the most attractive fuel cells for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among the fuel cells currently on the market.
The fuel cell stack included in the PEMFC includes a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a separator. The MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The separator functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
A fuel cell stack also includes a manifold block for forming an inlet flow field and an outlet flow field of the fuel cell stack, as a kind of flow field member which allows gas and coolant before and after the reaction to flow in and out of the fuel cell stack. The manifold block has a long and complicated internal flow field through which gas and coolant passes. When a plurality of stack modules are mounted in a fuel cell vehicle, the manifold block attached to the outside of the stack module serves to uniformly supply reactant gases (air and hydrogen) and coolant to each stack module.
To manufacture such a manifold block, a method for manufacturing the manifold block using an aluminum casting process and forming an oxidation layer for improving corrosion resistance on external and internal flow fields of the manifold block has been used. In such a method, the oxidation layer may be formed by plasma electrolytic oxidation (PEO) or anodizing, in which the die-cast manifold block is immersed in an electrolyte made of an inorganic material as a main ingredient. Electricity is then applied thereto, thus electrochemically forming the oxidation layer.
Referring to FIG. 1, after a manifold block 3 is located in an electrolyte bath 1 containing an electrolyte, a positive electrode is connected to the manifold block 3, a negative (−) electrode 5 is connected to electrolyte, a positive (+) electrode 4 is connected to the manifold block 3, and oxygen (air) is supplied to the electrolyte, thereby forming an oxidation layer on the surface of the manifold block 3. Here, a bubble generator 2 generates bubbles in the electrolyte bath 1 so that a sufficient amount of oxygen, which is required for the formation of the manifold block, can be supplied. This bubble generator, accordingly, supplies a large amount of oxygen to the electrolyte through the generated bubbles.
However, according to such a conventional method, while the oxidation layer can be easily formed on the outside of the manifold block and at the inlet of the internal flow field, it is very difficult to supply the power required for formation of the oxidation layer to the inside of the internal flow field. That is, it is necessary to supply sufficient oxygen required for the oxidation reaction so as to uniformly form the oxidation layer over the entire surface of the internal flow field. However, even when the bubbles are generated by the bubble generator provided in the electrolyte bath, sufficient oxygen is currently not supplied to the inside of the internal flow field. Accordingly, when the oxygen present in the electrolyte of the internal flow field is exhausted as the formation of the oxidation layer proceeds, the oxidation layer is no longer formed, which makes it very difficult to form the oxidation layer on the inside of the internal flow field. Thus, only the inlet of the internal flow field forms an oxidation layer in the conventional method.
FIG. 2 is a schematic diagram illustrating a region where no oxidation layer is formed. Referring to FIG. 2, while the oxidation layer can be formed with a length of about two times the inlet width B of the flow field, the oxidation layer is not formed in the other region (L−(2×B+2×B)). As such, the oxidation layer is not formed uniformly over the entire surface of the long and complicated internal flow field of the manifold block. Thus, this issue needs to be address in order to provide sufficient oxidation on the manifold block.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.